This invention relates to water purification and more particularily to the provision of ultrapure water substantially free of both organic and ionic contaminants.
High purity water is required for many purposes, including use in analysis, medicine and biology. Standards for water purity for various uses have been established by a number of professional organizations. The American Society for Testing and Materials (ASTM), for example, classifies purified water into Types I, II, III, and IV, based upon maximum allowable impurities. One measurement commonly employed is specific resistivity of the water in megohm/cm. at 25.degree. C., a measure of ionic contamination. Pure water has a theoretical resistivity of 18.2 and water can be provided approaching this value. Non-ionic contaminants such as organic materials and particulates are monitored by other analytical techniques and maximum values are sometimes specified.
Water is purified by a number of techniques, often used in combinations for the highest purity. These techniques include filtration, single or multiple distillation, sorption and ion exchange. Water initially treated by distillation or reverse-osmosis filtration is often "polished" or further purified by passage through activated carbon beds to sorb residual materials, principally organics, mixed beds of anion and cation exchange resins to remove residual ions, and then finally filtered through a microporous filter to remove residual particulates. Water of 18 megohm/cm. resistivity and low organic content can be thus provided.
Activated carbons contain ionic impurities, and occasionally collect microorganisms, which are released into the water. They are therefore normally used prior to treatment of the water with mixed ion exchange resins which remove the ions released. These resins, however, are organic and release trace quantities of organic contamination into the water. Such contamination is usually small, measured in the parts per billion range, and presents no undue problems for many uses. However, for some applications, for example trace organic analysis by high performance liquid chromatography (HPLC), such contamination can produce significant interference.
The need for improved techniques to reduce organic contaminants for critical applications is described in an article by Poirer and Sienkilwicz, entitled "Organic-free Water", American Laboratory, December 1980, pages 69-75. This article describes a device utilizing oxidation by ultraviolet light to reduce organics. While apparently effective, this treatment is relatively expensive and restricts output.